There are generally two types of active fibers used to couple light from laser diodes:

Single-mode fiber: The core is typically a few microns (eg, about 6 µm core at 1 µm wavelength, and about 9 µm core at 1.5 µm wavelength).

Multimode fiber: Larger diameter to handle higher levels of optical power. Standard versions have core diameters of 62, 100, 200, 400, 800 or even 1000 µm. The smaller the diameter, the easier it is to focus the light from the fiber onto a small spot using a lens or microscope objective.

their difference of Single-mode fiber and Multimode fiber

Polarization-Maintaining Fiber: Single-mode laser diodes can be standard fiber (SMF) or polarization-maintaining fiber (PM). In the latter, the fiber has a special cladding structure that maintains the polarization of the light over the entire length of the fiber.

Single Mode Fiber Coupled Laser Diodes

Single-mode fiber-coupled laser diodes This type of laser diode is usually assembled in a “butterfly” package that contains a TEC cooler and a thermistor (today’s trend is toward smaller form factors). Single-mode fiber-coupled laser diodes are typically capable of output powers ranging from a few hundred milliwatts to 1.5 watts.

There are several other package styles on the market. For example, the following DIL packages are often encountered in the telecom market, or coaxial packages, typically less than 10mW.